Grinding for Toolmaking

GRINDING FOR TOOLMAKING

All about grinding cold-working, hot-working, high-speed and mold-making steels, including PM high-performance materials MATERIALS FOR TOOLMAKING

Contents Introduction 3 Tool steels (including HSS and PM) 4 Grinding wheel composition 8 Abrasive 9 Grit size 10 Hardness 11 Structure 11 Binder 11

How grinding wheels work (grinding parameters) 12 Specific material removal rate Qw‘ 12 Speed ratio qs 13 Wheel cutting speed VC 13 Workpiece speed VW 14 Grinding wheel infeed ae 14 Change in wheel diameter 15 Contact area 16

Grinding problems and countermeasures 18 Grinder 20 Coolant 20 Dressing 21 Practical example 22 Recommendations for grinding BÖHLER tool steels and high-speed steels 26

2 Introduction Figure 1: Some of the links in the grinding process – The high alloy content of tool steels often makes them more THE WEAKEST LINK DEFINES THE difficult to grind than other steels, such as structural steels. For PERFORMANCE LIMIT satisfactory results when grinding tool steels, the grinding wheel must be selected carefully. In order to select the right grinding wheel and grinding parameters, however, you need to know how Coolant grinding wheels work.

This brochure provides an overview of the structure of grinding Material Grinding wheel wheels, how they work, and the parameters that determine the quality of grinding results. It also includes recommendations for suitable grinding wheels for conventional and powder-metallurgi- Diamond (for dressing) cal BÖHLER tool and high-speed steels. Machine

etc. Clamping fixture

This brochure was produced in cooperation with Rappold Winterthur Technologie. Setup parameters

Rappold Winterthur Technologie GmbH St. Magdalener Straße 85 A-9500 Villach Phone: +43 (0) 4242 41 811-0 Fax: +43 (0) 4242 41 811-700

3 GRINDING CRACKS AND STRESSES

Tool steels (including HSS and PM) In order to obtain good performance when grinding high-alloy carbide tool steels, it is important to select the right grinding The alloy components of tool steels greatly influence their wheel. MICROCLEAN steels, for example, contain a great grindability. The range of BÖHLER tool and high-speed steels many tungsten, molybdenum and vanadium carbides. In order extends from low-alloy steels such as K245 to very high-alloy to cut through these carbides, an abrasive that is harder than steels such as S290 MICROCLEAN. corundum or silicon carbide is needed. For these steel sorts, therefore, boron nitride grinding wheels are mainly recommen- Grinding low-alloy tool steels seldom presents any difficulties. ded. MICROCLEAN steels can nevertheless be ground using When grinding high-alloy tool and high-speed steels with high corundum or silicon carbide, but the steel matrix surrounding carbide content, however, problems can easily occur. For this the carbides will be ground away while the carbides are torn reason, the grinding parameters must be selected carefully. out of the matrix. This causes high wear of the grinding wheel, The higher the wear resistance of a steel, the more difficult it along with a risk of poor grinding results. is to grind. The wear resistance of a material is determined by the basic hardness of the steel and the hardness, size and Grinding cracks and stresses proportion of the included carbides. Choosing the wrong grinding wheel or parameters creates a In order to increase the wear resistance of a tool steel, the significant risk of crack formation in the workpiece. Grinding steel is alloyed with carbide-forming elements, primarily tung- cracks are not normally as easy to see as in Figure 2. As a sten, molybdenum and vanadium. The carbon content of the rule, the part needs to be inspected under a microscope or steel must also be relatively high in order to form carbides. using a dye penetrant in order to see the cracks. Hot etching Only boron nitride and diamond are harder than all of the with 50% HCl at a temperature of 70°C and a soak time of 20 carbides present in a tool steel. Diamond, however, is not minutes can also be used to make grinding cracks visible. suitable for grinding steel.

The number and size of the carbides in a steel greatly influ- ence its grindability. The more carbides are present, and the larger they are, the more difficult grinding will be. For this reason, powder-metallurgical steels with smaller carbides are easier to grind than smelted steels of similar composition.

4 SCHLEIFRISSE UND SCHLEIFSPANNUNGEN

The root cause of grinding cracks can be explained as follows:

Almost all of the energy expended for grinding is converted into heat, some due to friction and some due to deformation of the material. If the right grinding wheel has been selected, most of the energy is dissipated in the chips in the form of heat, and only a small amount warms the workpiece.

If a tool steel or high-speed steel that has been hardened and tempered is ground incorrectly, the ground surface can become so hot that the tempering temperature of the steel is exceeded. This reduces the surface hardness. If the tempe- rature continues to increase, it may even reach the austeniti- zation point and cause rehardening . This rehardened surface zone consists of non-tempered martensite and residual auste- Figure 2: Grinding cracks nite, as shown in Figure 3. Very high stresses occur, leading to crack formation. Figure 4 shows the hardness profile in the Grinding cracks typically form perpendicular to the direction surface zone of an improperly ground tool made of BÖHLER of grinding and normally mean that the workpiece must be K110. Here improper grinding caused the rehardening descri- scrapped. Grinding cracks occur more easily in hardened tool bed above. Due to the non-tempered martensite, the surface steels than in non-hardened ones. A tool steel or high-speed has high hardness. Directly beneath the surface is a region steel that has been hardened but not tempered should never where the hardness is lower than the basic hardness of the be ground. Hardened tool and high-speed steels must always tool steel because the tempering temperature was exceeded. be tempered before grinding.

5 Improper grinding that leads to this transformed surface zone Grinding stresses can be reduced by repeated tempering after can very often be detected by discoloration (burns) burns on grinding. In this case, the tempering temperature should be the ground surface. To prevent burns and grinding cracks, the about 30°C lower than the last tempering temperature in order temperature of the ground part must be kept low, for example to prevent loss of workpiece hardness. Another way to reduce by sufficient cooling. Properly dressed grinding wheels that grinding stresses is to peen the ground parts (see Figure 5.) cut the tool steel with sharp cutting edges must also be used, as otherwise excessive heat will be generated by friction.

The residual austenite content of a hardened and tempered tool steel or high-speed steel can have a substantial influence on grinding results. A high level of residual austenite increases the risk of crack formation during grinding. For most grinding processes, residual stresses arise in the ground surface zone.

These stresses are generally greatest just beneath the surface and can cause permanent deformation of the ground part, especially when grinding thin workpieces. The examples in Figures 4 and 5 show that the greatest risk of crack formation comes from improper grinding. Tensile stresses in the surface zone cause cracks in the material when they exceed the ten- sile strength of the steel. The risk of crack formation is lower with proper grinding – the surface stresses are compressive, Figure 3: Rehardened surface zone which can increase the durability.

6 Figure 4: Surface zone hardness profile – 1.2379

1000

800

600

SIC; no cooling

Microhardness (HV 0.05) Microhardness 400 SIC; with cooling CBN; no cooling CBN; with cooling

a = 100 µm, Vt = 2,4 m/min 200 0 100 200 300 400 500 600 700 800 900 1000 Distance from machined surface (µm)

Residual stresses after surface grinding of BÖHLER K110

Tensile stress

Improper grinding + Conventional grinding Electropolishing Proper grinding Improper grinding and sandblasting 0 Residual stress

–

Compressive stress

0 Edge depth

7 GRINDING WHEEL COMPOSITION

Grinding wheel composition

In principal, a grinding wheel consists of: Certain specialty grinding wheels, such as those with metal- ■ Abrasive bonded diamonds, have no pores. The composition and ■ Binder ratio of the components listed above determine the grinding ■ Pores properties of a wheel. The composition of a grinding wheel is defined by a standardized designation approved by ISO. It consists a defined sequence of letters and digits that indicate grits the abrasive, the grit size, the hardness and the binder.

Example: A46 H V

binder A Abrasive 46 Grit size pores H Hardness V Binder

Figure 6: The arrangement and ratios of abrasive grits, pores and binder determine the action of a grinding wheel.

8 Abrasive

There are currently three basic types of abrasive used for In order to take full advantage of the high performance grinding tool steels and HSS: potential of this abrasive, Winterthur has developed a perfectly a) Corundum Designation A tuned binder that fully exploits the self-sharpening properties b) Silicon carbide Designation C of the abrasive. c) Boron nitride Designation B a) Corundum Corundum is most often used as an abrasive for grinding iron White fused White fused Sintered corundum corundum corundum and steel and is available in various forms. It can be alloyed with other oxides, most commonly with titanium oxide. e.g. 31A – Mixture of normal, semifriable and fused white corundum 57A – Pink fused corundum pores 54A – White fused corundum with green binder binder

grits Unfortunately the color of the grinding wheel does not always indicate the type of abrasive, as some manufacturers dye Total Macro- Micro- their abrasives and binders. In recent years, sintered corund- splintering splintering splintering um (e.g. 93A), a new type of corundum with a fine crystalline structure has been developed. It gives the abrasive particularly Figure 7: Comparison of fused white and sintered corundum good self-sharpening properties. The pressure created during (microcrystalline grit structure) grinding causes microsplintering, which continuously produ- ces new, sharp cutting edges.

9 GRINDING WHEEL COMPOSITION

Properties of sintered corundum: c) Cubic boron nitride (CBN) Cubic boron nitride is produced similarly to synthetic dia- ■ High erosion rates, short cycle times monds. This abrasive is primarily used for grinding high ■ Long wheel life carbide content tool steels and high-speed steels. One dis- ■ Consistent grinding performance and low grinding forces advantage of boron nitride is its high price, but its price-to- ■ Good dimensional stability performance ratio compensates for this. ■ Cooler grinding due to matched binder and structure ■ Longer dressing intervals due to long service life Winterthur offers easily profiled CBN with normal and porous structures. The exceptional thermal conductivity of CBN b) Silicon carbide (SiC) ensures cool grinding, greatly reducing potential surface zone Silicon carbide is most often used as an abrasive for grinding damage. The tremendous wear resistance of CBN abrasive gray cast iron, carbides, plastic, glass, nonferrous metals and enables substantial increases in production and cost savings. austenitic stainless steels, although it can also be used for hardened, high-alloy tool steels up to 65 HRc. Winterthur nomenclature, cubic boron nitride: 3B 126 P 5 V C 100 Two types of silicon carbide 3B coar- soft 1 ceramic CBN There are two types of silicon carbide: black silicon carbide 5B se N to binder concentration (designated C) and somewhat harder green silicon carbide 32B 213 to 18 low (designated 11C), which is more brittle than the black type. etc. 126 S 50 to 91 hard 200 Winterthur nomenclature, conventional abrasive: to high 54 A 80 H 15 V P MF 604W 46 11 A coar- soft 1 cera- artificial L bond fine 31 C se A to mic porosity M type Figure 9: Grade parameters for grinding wheels 54 S 14 to 10 binder H 93 EA to Z F 11 etc. RA 500 hard G to fine Grit size 20 Abrasive grit size is of very important for selecting the correct Figure 8: Extended grade designation for grinding wheels: porosity 1–10 = normal structure; greater than 11 = high porosity grinding wheel. Grit sizes are classified by an international grit number (per FEPA.) Grit numbers F8 (coarse) through F1200 (superfine) correspond to the mesh count of a screen one inch on each side.

Coarse grits are used for high-throughput grinding (high rate of material removal), but this produces a rougher surface. Fine grits are used to produce a fine surface, for example when high edge stability is required.

Figure 10: Image of F12 grit size

10 Figure 10a: Ideal ceramic binder bridge

The surface roughness of the ground workpiece is not solely This artificially increased porosity is expressed by structure dependent on the grit size of the grinding wheel. The sharp- numbers from 11 to 19. Grinding wheels with structure ness of the wheel, the binder and the hardness of the wheel numbers 10 and 20 (porous) are produced using a special also affect the quality of the ground surface. For diamond and casting process. boron nitride grinding wheels, European manufacturers indi- cate the grit size by the grit in µm, in contrast to conventional Binder abrasives. The grinding grits are held together by a binder. The following Hardness binders are used to securely bond the grits in the microstruc- ture of the grinding wheel: The term “grinding wheel hardness” does not refer to the Ceramic Designation V grinding grit; rather, it indicates the resistance of the binder to Synthetic resin Designation B break-out of the grinding grit. In a softer wheel the grinding grit can break loose more easily than in a harder wheel. The Ceramic binder hardness is largely determined by the proportion of binder. Modern ceramic binders at Winterthur largely consist of syn- The hardness of a grinding wheel is indicated by the letters thetic engineered glasses known as low-fired binders. They A–Z, where A is the softest and Z the hardest. are immune to chemical effects and have unlimited shelf life. Structure However, abrupt changes in temperature or impacts should be avoided. Every grinding wheel has a natural porosity. At Winterthur this is expressed as a structure number in the range of 1 to 9, Ceramic bonded grinding wheels are most often used for known as normal structure. The greater the structure number, grinding tool steels. Synthetic resin is used as a binder for the more porous the grinding wheel. The natural porosity of a grinding wheels that run at high circumferential speeds. wheel can be increase artificially by adding artificial pore gene- rators to the mixture.

11 11 HOW GRINDING WHEELS WORK

How grinding wheels work (grinding parameters)

Grinding is a cutting process in which the grinding grits form Specific material removal rate Qw’ the cutting edges. The same principles apply to grinding as The specific material removal rate, known by the abbrevia- to other cutting processes, although certain factors make it tion Q ‘, describes the ability of a grinding wheel to remove necessary to consider the theory of grinding from a slightly w material in mm3 per mm of wheel width and per second. This different perspective. allows direct comparison of various grinding processes in order to evaluate the actual removal rate. The parameter Q ‘ These factors are: w is often used as the basis for calculating the infeed depth ae ■ The cutting tool has an irregular cutting geometry – and advance feed rate V . the arrangement of the grinding grits is irregular. W

■ The cutting geometry is variable – the action of an abrasive grinding tool includes a certain amount of self-sharpening, in which dull grits partially or completely break out to expose new grits.

■ Negative cutting angle – the irregular and blunt shapes of the grinding grits mean that the cutting angle is often negative.

■ A very large number of cutting edges

■ Very high cutting speed – the typical cutting speed (35 m/sec = 2100 m/min) for precision grinding is well above the typical speed for other cutting processes.

■ Very small chips. which means very small cutting depth for each cutting edge.

Figure 12: Graphic representation of Qw ■ Self-sharpening wheels – the pressure during grinding causes microsplintering, which continuously produces new, sharp cutting edges. Typical values and applications

Reciprocal flat grinding: ■ 1.5 mm3/mm/s Finishing ■ 2.0 mm3/mm/s Hardened high-alloy steel ■ 3.0 mm3/mm/s Good target value ■ 5.0 mm3/mm/s High target value ■ 10.0 mm3/mm/s High-performance process Binder ■ Material removal rates above 10 mm3/mm/s are typically seen in deep grinding processes

Grit Deep grinding: Chip ■ 7.0 mm3/mm/s Hardened high-alloy steel ■ 10.0 mm3/mm/s Minimum target value ■ 3 Tangential force Ft 20.0 mm /mm/s Good average value

Workpiece CD deep grinding: ■ 20.0 mm3/mm/s Minimum target value Normal force Fn ■ 50.0 mm3/mm/s High-performance process

Figure 11: Chip formation during grinding (highly schematic). Cutting angles are normally negative.

12 Speed ratio qs Various speed ratio ranges: The speed ratio qs is an important indicator of whether a grinding process is running optimally. This factor is the ratio Not Flat Thermally critical Creep feed used grin- range grinding of the circumferential speed of the grinding wheel to the ding workpiece speed.

10 50 120 1000 10000 circumferential speed V (m/s) x 1000 x 60 c Figure 14: Graphic representation of the critical range qs = workpiece speed Vw (mm/min.) Applications: Flat and external diameter grinding: Roughing: 60 to 80 Finishing: 80 to 120

qs < 50: risk of chatter marks qs > 120: risk of grinding burns

Creep feed grinding:

qs > 1000 to 10 000 qs < 1000: risk of grinding burns

The workpiece or bed speed VW can easily be calculated by specifying the qs value and using the circumferential speed of

Figure 13: ls’ = Contact arc length of single grit the grinding wheel. Rt = Surface roughness Vc = Circumferential speed Vw = Workpiece speed

Wheel cutting speed Vc Increasing vc: Reducing vc: ■ More cutting edges ■ Fewer cutting edges The circumferential speed of a grinding wheel has a direct engaged per unit time engaged per unit time influence on the number of cutting edges engaged in chip ■ Finer chips ■ Coarser chips cutting. For example, if the speed is doubled then twice as ■ Lower loads on ■ Higher loads on many grinding grits cut into the workpiece per unit time. individual grits individual grits This reduces the self-sharpening effect. so the grinding wheel ■ Less tendency for ■ Greater tendency for ultimately acts harder and produces a finer surface finish, but single grits to split single grits to split it also carries the risk of surface burning.

The wheel acts harder. The wheel acts softer. By contrast, reducing the circumferential speed of a grinding wheel results in thicker chips, causing the grinding wheel to act softer. Generally, both the circumferential speed and the workpiece speed are increased in order to increase the Changes to circumferential speed material removal rate.

13 A typical safety limit for ceramic-bonded wheels is 40 m/s. Grinding wheel infeed ae Some grinding wheels, however, are approved for circumferen- The grinding wheel infeed depends on the type of wheel, tial speeds up to 100 m/s. An appropriate circumferential speed the stability of the machine, and/or how firmly the workpiece for synthetic-resin-bonded boron nitride wheels is 35 to 40 m/s. is clamped. For ceramic-bonded boron nitride wheels, a cutting speed of 45 to 63 m/s is often required. Guidelines for infeed using conventional wheels: Rough grinding: ~ 0.05 mm/pass Workpiece speed V w Finish grinding: ~ 0.005 – 0.010 mm/pass The grinding characteristics of a wheel can be altered by changing the workpiece speed. Increasing the workpiece The infeed rate should be reduced when grinding with boron speed makes the wheel act softer, and decreasing the speed nitride wheels. If wheels with sintered corundum are used, makes the wheel act harder. the infeed should be set a little higher than the guidelines in order to increase the grinding pressure and obtain good self-sharpening.

Changing the workpiece speed Changing the infeed depth

Increasing vw: Reducing vw: Increasing ae: Reducing ae: ■ Increased material removal ■ Reduced material removal ■ Increased material removal ■ Reduced material removal ■ Coarser chips ■ Finer chips ■ Coarser chips ■ Finer chips ■ Higher loads on ■ Lower loads on ■ Higher loads on ■ Lower loads on individual grits individual grits individual grits individual grits ■ Greater tendency for ■ Less tendency for ■ Greater tendency for ■ Less tendency for single grits to split single grits to split single grits to split single grits to split

The wheel acts softer. The wheel acts harder. The wheel acts softer. The wheel acts harder.

14 ARBEITSWEISE DER SCHLEIFSCHEIBEN

Change in wheel diameter

Increasing ds: Reducing ds: ■ Contact area Ak between ■ Smaller contact area wheel and workpiece ■ Grinding forces remain increases unchanged ■ Grinding forces remain ■ Higher loads on practically unchanged individual grits ■ Lower loads on ■ Greater tendency for individual grits single grits to split ■ Less tendency for single grits to split The wheel acts harder. The wheel acts softer.

15 Contact area

The actual material removal takes place at the contact surface self-sharpening of the wheel, it must have a softer compositi- between the grinding wheel and the workpiece. With a greater on than for external cylindrical grinding of a similar workpiece. contact area, a larger number of cutting edges are engaged The contact width can be the width of the grinding wheel, for in chip cutting. This makes the chips smaller and the specific example with plunge grinding. If necessary, the contact width forces lower. The opposite is true for a smaller contact area. can be reduced by dressing the grinding wheel. This reduces the contact area and causes thicker chips and higher forces The contact length depends primarily on the grinding process. on the abrasive grits, so the wheel acts softer. It also depends on the diameter of the grinding wheel, the cutting depth, and the workpiece dimensions. Differences in The number of cutting edges has a substantial effect on the contact length are mainly significant when selecting a suitable grinding process. A larger number of cutting edges per unit wheel composition for a particular grinding process. area means that the cutting work is distributed over a larger number of abrasive grits, so the specific forces are lower. The For example, if a grinding wheel with a diameter only a little grit size of the abrasive also influences the number of cutting smaller than the internal diameter of the workpiece is used edges at the contact surface, causing fine-grit grinding wheels for internal cylindrical grinding, the contact length is very long, to act harder. leading to a low cutting force on the abrasive grit. For proper

16 External cylindrical grinding Internal cylindrical grinding (centerless grinding)

Flat grinding Face grinding

Figure 11: Contact lengths of various grinding processes

17 GRINDING PROBLEMS AND COUNTERMEASURES

If the grinding wheel acts too hard or too fine: Indicator Countermeasure ■ Wheel wears down quickly ■ Use a softer wheel or more ■ Too little bite open structure ■ Burning ■ Select a coarser grit ■ Tendency to chatter ■ Reduce the wheel speed • Wheel needs to be ■ Increase the feed, infeed dressed often because and/or workpiece speed it no longer cuts

If the wheel acts too soft or too coarse: Indicator Countermeasure ■ Wheel loses its profile ■ Select a harder wheel ■ Wheel does not maintain or tighter structure its dimension ■ Select a finer grit The most common grinding problems and their ■ Wheel needs to be ■ Increase the wheel speed countermeasures are: dressed often because it ■ Reduce the feed, infeed no longer holds its shape and/or workpiece speed a) Indicator: Thread pattern with pitch matching the table • Wheel consumption is feed is visible on the surface of the workpiece too high Correction

■ The surface of the grinding ■ Monitor the dressing tool wheel is not parallel to the and thermal conditions motion of the bed (offset, thermal effect or dressing tool wear.)

b) Indicator: Surface has spirals, diagonal marks, or a c) Indicator: Workpiece surface shows irregularly distributed regular pattern short, comma-shaped scratches

Cause Correction Cause Correction ■ Defective dressing process ■ Dress the wheel in one ■ Particles floating in the ■ Clean the cover produces out-of-round direction only; reduce coolant or clogged wheel ■ Check the filter wheel, which transfers the dressing feed if necessary ■ Select a more open error to the workpiece wheel structure

18 d) Indicator: Chatter marks parallel to the workpiece e) Indicator: Spiral or localized yellow or brown surface discoloration

Cause Correction Cause Correction ■ ■ Grinding wheel imbalance Check workpiece fixturing ■ Overheating in the ■ Increase the dressing ■ Vibration of the workpiece ■ Check wheel for grinding process infeed, speed ratio < 120, or machine imbalance or runout Insufficient coolant flow? increase coolant flow ■ ■ Wheel/workpiece speed Never let coolant run Speed ratio too high ■ Use coolant with mineral ratio too low (qs < 60) across a wheel that is (qs >= 120)? oil content standing still ■ Wheel dressed too finely? ■ Select a softer wheel or ■ Use a steady rest? ■ Wheel too hard more open structure (check) qs? (check)

f) Indicator: Grinding with dull wheel, mirror finish or g) Indicator: Facets occur parallel to the workpiece axis uneven areas from premachining remain intact (partial or over the entire circumference)

Cause Correction Cause Correction ■ Wheel dressed too finely, ■ Increase dressing infeed ■ Interference from inside ■ Eliminate the interference clogged, or grit too fine ■ Use new single-crystal or outside the machine diamond (e.g. coolant pump, blower, ■ Use a more open forklift, punch presses, etc.) (porous?) or coarser wheel

19 GRINDER AND COOLANT

Grinder The grinding process and the machine greatly influence the Three types of cutting fluids are primarily used selection of the wheel composition. for grinding: Water-based A grinder should be as rigid as possible, so that it can work These are fluids consisting of water with synthetic additives. with high grinding pressure. This high grinding pressure is They have good cooling capacity, but their lubrication advantageous when using a sintered corundum, for example, properties are not as good. as it is ideal for fostering the self-sharpening effect. The rigidity of the grinder and the type of fixturing also help to determine Emulsions the selection of the grinding wheel. These consist of water with 3–5% highly emulsified oil additive. Lower concentrations cause the pH value to drop, If the machine is not rigid enough, then a softer wheel or and odors or corrosion can occur. Emulsions with too high a smaller contact surface between the wheel and workpiece concentration can cause wheel clogging during grinding and should be selected in order to ensure the required degree of loss of material removal performance (reduced flushing effect). self-sharpening. The spindle speed (spindle drive power) of Foaming can also occur if the concentration is too high and the grinder also affects the wheel selection. the water hardness is too low. Emulsions are currently used for most grinding processes due to their low environmental impact and adequate performance. Coolant Cutting fluid is used for grinding, just like all other cutting Cutting oils processes. This coolant is primarily needed to These are normally based on a mineral oil with additives. Cutting oils have very effective lubrication properties, but their ■ Cool the workpiece cooling performance is not as good. ■ Act as a lubricant to reduce friction between the chips, the workpiece and the grinding wheel ■ Prevent edge-zone effects on the workpiece ■ Provide heat dissipation from the contact zone ■ Flush out chips from the contact area ■ Ensure consistent abrasive properties

20 Dressing General: ■ In general, the dressing tool infeed should be low Dressing the grinding wheel results in precise runout and the (0.002 to 0.03 mm), as otherwise the microstructure correct geometric shape. Dressing is also used to produce the will be damaged. desired effective roughness and to influence cutting capacity. Controlled changes to dressing conditions can change the ■ To adjust the effective surface roughness, the dressing topography of the wheel, which has a substantial influence on infeed rate vd should be varied (higher vd = greater the grinding process and the desired result. roughness, and vice versa).

Dressing can be done in a stationary or rotating process. Fused red corundum, abrasive fabrics Important notes: or mixed corundum can be used for stationary dressing. Rotating dressing wheels running in the ■ Always use appropriate cooling when dressing. same or opposite direction are used for rotational Diamonds are very sensitive to heat! dressing. For detailed information on dressing parameters, please contact Rappold Winterthur ■ NEVER traverse the wheel without infeed (ad = 0), Technologie. because this will dull the wheel. Dressing terminology ad = infeed depth of the diamond tool (mm) bd = effective width of the diamond tool (mm) ns = grinding wheel speed (rpm) sd = diamond tool infeed per wheel revolution (mm/rev) Ud = overlap (number) Vd = diamond tool infeed rate (mm/min)

21 AUS DER PRAXIS

PRACTICAL EXAMPLE

Determining the optimal abrasive for electroslag-remelted BÖHLER K340 ISODUR and powder-metallurgical BÖHLER K390 MICROCLEAN. IIn a practical test, external cylindrical grinding was performed on BÖHLER K110 and BÖHLER K340 ISODUR (electroslag remelted 8% Cr cold-working steel with optimized toughness) and BÖHLER K390 MICROCLEAN (powder metallurgy)

The rods (Ø 80 mm, length 330 mm) were heat treated at BÖHLER, and all grades were ground after annealing to a hardness of less than 62 HRc and greater than 62 HRc. The abrasives were provided by Winterthur Technology. The grin- ding tests were performed in cooperation with the Institute for Manufacturing Engineering at the Technical University of Graz. The goal of the practical tests was to determine the optimal abrasive for the steel grades listed above (BÖHLER K110, BÖHLER K340 ISODUR and BÖHLER K390 MICROCLEAN).

22 The following grinding parameters were used:

Grinding process External cylindrical grinding, reciprocal

Machine: Kellenberger Kelvaria UR175/1000 (7,5 kW)

Hardness: Through-hardened (approximately 62 HRc) Wheel designation: 57A80 H8V300W-50 m/s - Pink fused corundum 64A80 H8V300W-50 m/s - Mixture of pink fused and single-crystal corundum 93N80 H8V902W-50 m/s - NanoWin® (special corundum) 93A80 H8V601W-50 m/s - Mixture of white fused and sintered corundum 11C80 J15VPLF-50 m/s - Silicon carbide 32B126 O15CVPMFC75 - CBN Parameters: The same settings were used for each grinding wheel in the tests. • Same direction: yes • Wheel speed vc = 40 m/s • Workpiece speed nw = 200 rpm • Infeed ae = 0.008 mm on both sides • Addition = 1.0 mm on the diameter Dressing: With PKD nonwoven • Wheel speed vc = 40 m/s • Dressing speed vd = 200 mm/min • Dressing infeed ad = 0.02 mm on both sides Coolant: Blasocut Emulsion 3.5%, fully synthetic

23 The following diagram shows a summary Cost effectiveness: of the results of the best grinding wheel Cost effectiveness is evaluated as the least wheel wear grades for the different steel grades: relative to the maximum material removal rate (G factor) for Performance limits each steel grade and hardness.

■ In the annealed state (~56 HRc), pink fused corundum 30 (57A) is best

■ 25 With material hardness below 62 HRc, the 93N material (NanoWin®) was the most suitable universal 20 wheel, closely followed by the sintered corundum variant 93A80 H8V601W 15 ■ The harder the material, the more cost effective the G factor CBN wheel becomes. The CBN wheel is by far the 10 best option for full hardness (> 62 HRc) with respect to the G factor, but is several times more expensive 5 than conventional ceramics.

0 annealed <62 HRc >62 HRc >62 HRc 57A80 93N80 93A80 CBN H8V300W H8V601W H8V601W

K110 K340 K390

24 The following tables contain details for grinding tools, molds The BÖHLER MICROCLEAN steels set new standards for and dies made from BÖHLER hot-working, mold-making, high performance in terms of toughness, wear resistance, cold-working and high-speed steels. These recommendations compressive strength and corrosion resistance. were developed jointly by BÖHLER Edelstahl and Rappold Winterthur Technologie. An extensive range of cold-working, mold-making and high-speed steels gives our customers a clear competitive Grinding powder-metallurgical materials advantage. BÖHLER has continued to refine the manufacturing process for powder-metallurgical high-speed steels and tool steels. Grinding recommendations are also provided for powder- With the world‘s most modern system in Kapfenberg, the third metallurgical BÖHLER steels, which are more difficult to grind generation of MICROCLEAN materials is being produced with because of their high alloy content. even better performance characteristics.

25 RECOMMENDATIONS FOR GRINDING BÖHLER TOOL STEELS AND HIGH-SPEED STEELS

BÖHLER Condition External cylindrical Internal cylindrical Centerless Flat Profile and creep grade grinding grinding grinding grinding grinding Medium alloy steel

K245 annealed 57A80 J7V300W 54A80 H9V904W Throughfeed 31A80 L6V301W 64A60 H15VP300W 54A80 H15VPMF904W K455 57A60 K5V300W Plunge 31A120 L6V301W K460 K510 hardened 54A60 J7V904W 93A80 H13VP601 Throughfeed 54A80 L6V604W 93A60 F15VPH601W 93A80 F15VPH601W K600 54A80 H8V904W 93A80 L6V601W K605 Plunge 54A180 L6V604W K720 93A120 L6V601W W300 W302 W303 W320 W321 W350

W360

W400

W403

W600

M200 prehardened 54A60 J7V904W 54A80 H15VPMF904W Throughfeed 31A80 L6V301W 54A60 H15VPMF904W 54A80 H15VPMF904W M238 Plunge 31A120 L6V301W M261 M268 hardened 54A80 H8V904W 32B91 P8CV600C100 Throughfeed 54A80 L6V604W 54A60 F15VPH904W 54A80 F15VPH904W M300 93A80 H8V601W ev. 93A80 H13VP601 Plunge 54A180 L6V604W

M303

M314

M315

26 BÖHLER Condition External cylin- Internal cylin- Centerless Flat Profile and creep grade drical grinding drical grinding grinding grinding grinding High alloy steel

K100 annealed 57A80 J7V300W 64A80 H8V300W Throughfeed 31A80 L6V301W 54A60 H15VPMF904W 54A80 H15VPMF904W K110 64A80 H8V300W 54A80 H15VPMF904W Plunge 31A120 L6V301W K305 K306 hardened 93A80 H8V601W 93A80 H13VP601 Throughfeed 93A80 J7V601W 64A60 H15VP300W 93A80 F15VPH601W K353 < 62HRc 93N80 H8V902W 11C60 H15VP Plunge 93A120 J7V601W 54A80 F15VPH904W K340 ev. 11C120 K4V

K360 hardened 32B91 P5V600C100 32B91 P8CV600C100 Throughfeed 32B91 P8CV600C100 93A60 F15VPH601W 93A80 F15VPH601W S400 > 62HRc ev. 93A80 H8V601W ev. 93A80 H13VP601 Plunge 32B126 N5CV800C100 32B126 Q15CVPMF600C75 S500 S600 S700 S404 S607 S630 S705 M310

M333

M340

PM grades

S290 annealed 57A80 H8V300W 54A80 H15VPMF904W Throughfeed 54A80 J7V904W 54A60 H15VPMF904W 54A80 H15VPMF904W Plunge 54A120 J7V904W S390

S590 hardened 93N80 H8V601W 93A80 H13VP601 Throughfeed 93A80 J7V601W 64A60 H15VP300W 93A80 F15VPH601W < 62HRc Plunge 93A120 J7V601W 54A80 F15VPH904W S690

S790 hardened 32B91 P5V600C100 32B91 P8CV600C100 Throughfeed 32B126 P8CV600C100 93A60 F15VPH601W 93A80 F15VPH601W > 62HRc ev. 93A80 H8V601W Plunge 32B126 N5CV800C100 32B126 Q15CVPMF600C75 K390

K490

K890

M368

M390

Wheel grades: 93N... NanoWin, suitable for soft alloy components 31A... Mixture of normal, semifriable, fused white corundum 54A... Fused white corundum with recrystallized binder system 93A... Mixture of sintered corundum and white fused corundum 57A... Pink fused corundum, grit somewhat tougher than 11C... Green SiC 64A... Mixture of single-crystal and fused pink corundum 32B... Cubic boron nitride (CBN)

27 SPECIAL STEEL FOR THE WORLD´S TOP PERFORMERS

Provided by: BÖHLER Edelstahl GmbH & Co KG Mariazeller Straße 25 A-8605 Kapfenberg/Austria Phone: +43-3862-20-60 46 Fax: +43-3862-20-75 63 E-Mail: [email protected] www.bohler-edelstahl.com

The information in this brochure is non-binding and does not constitute an agreement; rather, it is for general informational purposes. This information is binding only if it is explicitly made a condition of a contract concluded with us. Measurement data has been obtained under laboratory conditions and may deviate from field results. No substances that are hazardous to health or to the ozone layer are

Printed on chlorine-free bleached paper having no pollution effects. Printed on chlorine-free used in the production of our products. BW 141 E - 09.2016 - 1.000 CD